Tap changing current regulator

ABSTRACT

A constant current control system for lighting applications, in particular airport series lighting arrangements, using a multitap transformer connected to the load effectively through backto-back thyristors which are selectively fired to connect the various taps to the load in order to regulate the current. A signal representative of the square of the current is used to produce the control signal which controls the firing point of the various thyristors. The control signal is a series of impulses produced by a multivibrator.

United States atet Legg et a1. 2 July 3, 1973 TAP CHANGING CURRENTREGULATOR 3,448,371 6/1969 Boymel 323/4 3,600,668 8 1971 o ldb 323 43.5s

[761 lnvemmsl Brian Legg, Coral Dnve- 3,469,176 9/1969 1126...??? 323/4Hamilmm Ontario; i Cowie, 3,061,828 10/1962 Hauck 323/6 ux 4517 Cott nwoD 3,596,172 7/1971 Harrison 323/45 Burlington, Ontario,.both of3,686,557 8/1972 Fulamura 323/24 X Canada Filed: June 12, 1972 Appl.No.: 262,122

Foreign Application Priority Data July 2, 1971 Canada 117,181

US. Cl 323/4, 315/219, 315/279, 315/307, 323/6, 323/24, 323/435 S,323/62 Int. Cl. G05f 1/20 Field of Search 315/196, 219, 251, 315/258,279, 307; 323/4, 6, 7, 24, 43.5 S, 45,

References Cited UNITED STATES PATENTS Mellott et al. 323/435 S PrimaryExaminer-A. D.- Pellinen Att0rney--R. H. Fox

[57] ABSTRACT A constant current control system for lightingapplications, in particular airport series lighting arrangements, usinga multi-tap transformer connected to the load effectively throughback-to-back thyristors which are selectively fired to connect thevarious taps to the load in order to regulate the current. A signalrepresentative of the square of the current is used to produce thecontrol signal which controls the firing point of the Variousthyristors. The control signal is a series of impulses produced by amultivibrator.

2 Claims, 2 Drawing Figures 2OA 22 22A 22B Patented July 3, 1973 2Sheets-Sheet 1 Patented July 3, 1973 2 Sheets-Sheet 8 TAP CHANGINGCURRENT REGULATOR BACKGROUND OF THE INVENTION This invention relates tothe regulation of power supplied to a load, in particular a serieslighting load such as a lighting arrangement conventionally used forairport lighting.

v In the prior control of airport lighting, it has been conventional touse saturable reactors to control the current supplied to theseries-arranged lamps. It is conventional in the operation of such lampsthat a short circuit be placed across the lamps when they are notoperative. When the supply is applied to this load in its short circuitcondition, it will be evident that, in the absence of regulation,prohibitively large currents would flow. However, in the prior art itwas usual, as has been previously indicated, to unsaturate the reactorunder these conditions and cause the short circuit to appear, so far asthe actual source of energy is concerned, as a highly reactive load. Thecurrent was then limited in accordance with the reactance to thepermissible current limit. The resultant power would be essentiallywholly reactive and would represent the source voltage times thereactive current in VARS. Such a reactive load is obviouslydisadvantageous particularly in view of power factor penalties which areassessable by suppliers and the excessively high line currents whensupplied by standby Diesel generator sets. It would therefore bedesirable to reduce this if possible. Saturable reactors also haddisadvantages in their slow response time, high losses and severewaveform distortion would SUMMARY OF THE INVENTION These and otherobjections are overcome in the present invention by utilizing solidstate switching devices, not only to regulate the current by controllingthe conducting angle of the solid state devices, but also selectively tosupply current from different taps of a supply transformer in accordancewith the current demand so that the wave shape is more closely relatedto sinusoidal, and also that in short circuit conditions a lower voltageis applied from the source, thus reducing the total VAR consumption ofthe system under these short circuit conditions.

A clearer understanding ofour invention may be had from a considerationof the following drawings, in which:

FIG. 1 is a schematic diagram of a system in accordance with ourinvention; and V FIG. 2 is a waveform diagram useful in explaining theoperation of the circuit.

DESCRIPTION OF THE PREFERRED I EMBODIMENTS As will be seen in FIG. I,the load transformer 10 is supplied from a source of 60-cyclealternating current at the available voltage asconvenient. The secondaryof this transformer is provided with several taps. Connected to one ofthese taps, designated 11, are a pair of thyristors 12 and 13 arrangedback-to-back. The other ends of these thyristors are connected togetherand to the primary of the output transformer 14. A similar pair ofthyristors designated 15 and 16 are connected to tap 17 and totheprimary of output trans former 14. To the output of the outputtransformer 14 is connected the load, consisting of a series of lamps18, 18a, 18b, etc., each supplied through its individual transformer 22,22A, 22B, etc. This load is connected to the secondary of thetransformer through a series of contacts 20a, 20b, 20c and 20d, all ofwhich are interrelated so that when 20a and 20b are closed, 20d and 200are open and conversely when 20d and 200 are closed, 20a and 2012 areopen.

A current transformer 19 is connected in series with the primary oftransformer 14 and the output of this transformer is applied totransducer 21. Transducer 21 is arranged to produce a positive potentialrepresentative of the square of the current passing through the currenttransformer 19. This signal is passed through resistor 24 to theoperational amplifier 28.

This operational amplifier, together with all similar operationalamplifiers in the system, is provided with the usual feedback circuitsand potentials to permit its normal operation, all of which are notshown. Also applied to the operational amplifier 28 is a referencesignal derived from potentiometer 26 which is negative in polarity andis applied .through resistor 27 to the'input of the operationalamplifier. This reference signal establishes the desired brilliance ofthe lamps 18. As will be seen, the input to theoperational amplifier 28is the sum of a positive and a negative signal, and therefore representsthe difference between the "output from transducer 21 and the reference,which may be referred to as an error. This error is integratedin'amplifier 28 which is not a simple operational amplifier, but is anintegrating amplifier. As a result, its output represents theaccumulated error. This output is applied to a further operationalamplifier 29, whose point of operation is established bythe biasprovided from potentiomet'er 30. When and only when the signal from theoperational amplifier 28 exceeds the bias potential, amplifier 29produces an output which is applied through resistor 33 to operationalamplifier 31.

A further signal is applied to operational amplifier 31 from the rampgenerator 32 which is provided with a reference frequency at 60 cycles,which corresponds tothe source. In response to this reference, the rampgenerator produces a linearly decreasing sawtooth wave, as

shown at C in FIG. 2, in phase synchronism with the waveform of thesource and having a duration of onehalf cycle. This ramp is appliedthrough resistor 34 to amplifier 31. Amplifier 31 is arranged so that itprovides at its output as long as the combination of the interatedsignal and the ramp is negative, a positive output.

The next portion of the circuit consists of an operational amplifierarranged as a multivibrator, controlled by the potential at the junctionof resistors 35 and.36.

nected a diode 44 and a Zener diode 45 in series. The

other input of the amplifier, that is, the non-inverting input, isconnected through resistor 48 to a potentiometer consisting of resistors46 and 47 which are connected between the output of the amplifier andground. This input terminal of the amplifier is also connected throughresistor 49 and diode 77 to the junction of resistors 35 and 36. Thesquare waves produced by the amplifier 37 functioning as a multivibratorare applied through resistor 38 to the pulse forming network which is asymmetrical circuit arranged to operated on alternate half-cycles andthereby provide the necessary controls to the gates of thyristors and16.

As will be seen, the resistor 38 is connected through resistors 50 and51 to the bases of transistors 52 and 53. The collectors of thesetransistors are connected to a source of positive potential at terminal54 through resistors 55 and 56. The collectors are also connected to thecollectors of switching transistors 57 and 58; the emitters of both ofthese transistors being connected to ground. The emitters of transistors52 and 53 are con nected to the bases of transistors 59 and 60. Thecollectors of these transistors are connected to the positive supplyterminal 54 through resistors 61 and 62 and load resistors 63 and 64.The output from the transistors is applied from the load resistors tothe primary of pulse transformers 65 and 66. The emitters of transistors59 and 60 are connected to ground. The bases of transistors 57 and 58are driven from switching transformer 67. One terminal of thistransformer is connected through diode 68 and resistor 69 to the base oftransistor 57. The base of transistor 57 is also connected to groundthrough r esistor76 and Zener diode 71. In a similar manner, theopposite terminal of transformer 67 is connected through diode 72 andresistor 73 to the base of transistor 58, which base is connected toground through Zener diode 74 and resistor 75. The centre point of thesecondary transformer 67 is connected to the positive supply terminal 54and the primary is provided with 60-cycle alternating current asindicated; this current being from the same source as other 60-cyclesupplies in the system. The junction point of resistors 61 and 63 andresistors 62 and 641 is by-passed to ground through capacitors 77 and 76respectively. The secondary of transformer 66 is connected between thegate and cathode of thyristor i6 and the secondary of transformer-65 isconnected between the gate and cathode of thyristor 1%.

The firing circuit just described is associated with thyristors 15 and16. A similar circuit is of course re quired in association withthyristors 12 and 13, which circuit corresponds exactly with that shownand utilizes, insofar as possible, common waveforms and components. Thebasis of differentiation between the circuits is the bias supplied tothe operational amplifier 28 and the corresponding amplifiers in furtherfiring circuits.

OPERATION Let us assume that a 60-cycle supply, as shown at A in FIG. 2,is provided for transformer w and the system is in operation providingcurrent to the load consisting oflamps 18, 18a, etc. with the switchesin the condition shown producing an effective load resistance as shownat time period t of curve B in FIG. 2. Waveform C in FIG. .2 is intendedto be illustrative only. It will be seen that the solid line representsthe ramp, while the dotted line represents the integrated output fromamplifier 29. The polarity of this latter should be disregarded however,since it effectively illustrates the zero potential level of thecombined signal indicating the point of transition from positive tonegative value. At the point of intersection, the potential at the inputof amplifier 3% changes polarity. it will therefore be seen that thepolarity of the output from amplifier 31 varies from negative topositive at a time determined by the combination of the ramp and theintegrated error signal.

Let us now consider the operation of the multivibrator consisting ofamplifier 37 and its associated components. Let us assume that theoutput terminal of amplifier 37 is in its most positive state and hasbeen in this state long enough to charge up capacitor 43 to a pointwhere the inverting input terminal of the amplifier connected tocapacitor 43 has just reached the potential of the non-inverting inputterminal connected to resistor 49. The output will suddenly decrease andthe output terminal of the amplifier will go to a negative valuedepending on its design. It remains in this condition until capacitor 43discharges through diode 41 and resistor 39. When capacitor 43 hasdischarged to a sufficiently low level, the amplifier once againreverses its condition and the output becomes positive. This reversal ofpositive to negative condition occurs repetitively as long as thenon-inverting input terminal of amplifier 37 is permitted to gopositive, and occurs at a periodicity determined by resistors 39 and 40.it will be noted that one resistor, i.e. 39, determines the dischargerate and another resistor 40 determines the charge rate because of theinclusion of diodes 41 and 42.

If, however, the potential at the junction of resistors 35 and 36 isnegative, the non-inverting input terminal to amplifier 37 is held at anegative potential. This negative potential at the non-inverting inputto amplifier 37 produces a negative potential at the amplifier output,which charges capacitor 43 to a negative potential through resistor 39and diode 41. The charge on capacitor 43 is limited by diode 44 andZener diode 45 so that the magnitude of the potential at thenon-inverting input is greater than that at the inverting input and themultivibrator action is arrested. In the absence of Zener diode 435, thecapacitor would continue charging until it reaches the amplifiersaturation voltage which would prevent the multivibrator resumingoperation in a normal manner. As soon as the output from amplifier 31goes positive, the multivibrator circuit is permitted to operate andproduces a series of pulses. These pulses continue as long as the outputfrom amplifier 31 is positive. The output pulses are applied aspreviously indicated through resistor 38 to the pulse forming network.

The operation of the pulse forming network is as follows.

Assuming a series of pulses is applied through resistor 38 to the baseof transistor 52 for example, this would normally cause transistor 52 tobecome conductive. However, it will be noted that transistor 57 isplaced between the collector and ground on transistor 52 so that iftransistor 57 is switched on, transistor 52 cannot be switched on. Theoperation of transistor 57 is determined by the switching circuit whichincludes transformer 67. It will be seen that the centre tap of thesecondary of this transformer is supplied with a positive potential. inaddition, let us assume that the secondary of the transformer isproviding positive potential to diode 6% through resistor 69 to the baseof transistor 57. This will cause transistor 57 to be conductive andtransistor 52 therefore can not be operative. Zener diode ii is providedto protect transistor 57 from the voltages produced by the combinationof the supply voltage from terminal 54 and the output from the secondaryof transformer 67. In order to provide rapid switching it is desirablethat the output from transformer 67 be large thus producing an abruptrise of voltage to a point where transistor 57 becomes conductive. Onthe other hand, it is not desired that large voltages be applied to thebase of the transistor and these voltages are instead absorbed throughthe Zener diode 71.

When the polarity of the secondary of transformer 67 reverses, anegative potential appears on its secondary. This causes transistor 57to be cut off and in this condition transistor 52 can become operative.With the pulses applied to its base through resistor 50, it then appliesa similar series of pulses to the base of transistor 59. This transistorin turn conducts a series of current pulses which appears across itsload resistor 63 and is applied to pulse transformer 65. These pulses ofthe form shown at C in FIG. 3 in turn are applied to the gate andcathode of thyristor causing the thyristor to commence conducting.

It will' be seen therefore that the point of conduction of the thyristoris determined by the time of conduction of transistor 52, which canoperate only during a partic ular half-cycle which is arranged to be theconductive half-cycle of thyristor l5, and. also during the period whichthe multivibrator circuit produces pulses, this period being determinedin time by the error signal and its point of coincidence with the rampproduced by the ramp generator. It will therefore appear that the pointof conduction of thyristor 15 is determined by the cur- In time period tthe dotted line cuts the ramp early in the cycle and causes thyristorsl2 and 13 to fire. Later in the cycle the dash-dot line alsocuts theramp and thyristors 15 and 16 are fired. The resultant voltage andcurrent waveforms are shown at D and E respectively and the pulse trainsused to fire thyristor 15 for example are shown at C.

In time period t the load resistance increases and more voltage isrequired to produce the same current. Thyristors 12 and 13 fire for thewhole cycle while thyristors 15 and 16 fire for a substantial portion ofthe cycle.

In time period t, the load resistance is zero (with contacts 20d and 200closed). The error signal is such that the amplifier associated withthyristors l5 and 16 is inoperative (due to its bias) and onlythyristors l2 and'l3 are fired. Since these are feeding a reactive loadthey conduct through the zero transistion. The series of pulses as shownat C' or a prolonged firing signal is' used to ensure thyristors turn onin this period.

It should be evident that this system as illustrated is only onespecific example of our invention. Only two thyristor pairs have beenshown on transformer 10, but

rent passing through current transformer 19. In a similar manner, theconducting period of thyristor 16 is also so determined. Thyristors 12and 13 are similarly controlled by a similar circuit, the onlydifference being the adjustment of potentiometer corresponding topotentiometer 30, which determines the thyristor to be connecteddepending on the current required. Under circumstances where the load isnot connected as shown, but instead is in its non-operative condition;that is, with-contacts 20a and 20b open, and contacts 20d and 20cclosed, a short circuit is applied to the secondary of transformer 14.This causes a current in the current transformer 19 whch rapidly risesto and exceeds the value established by potentiometer 26.

As shown by the dotted line in curve C of FIG. 2, the integrated errorsignal changes rapidly in a positive di rection so that the effectivepoint of intersection between the ramp and the integrated signal isdelayed to a point late in the cycle. Therefore, the effective potentialat the input of amplifier 31 only becomes negative late in the cycle andtherefore the point of conduction of thyristors l2 and 13 is late in thecycle. It will be noted that the dash-dot line which represents thepoint of conduction of thyristors l5 and 16 goes right off the graph.These thyristors do not conduct under this condition, and onlythyristors connected to the lower potential taps are fired. The variousconductive conditions are illustrated in FIG. 2

As the load resistance changes as shown at B, current errors occur whichwhen integrated by amplifier 28 and amplified by amplifiers such as 29,produce reference represented by the broken lines on C. The dotted linerepresents the reference used to control thyristors 12 and 13 while thedash-dot line represents the reference used to control thyristors l5 and16.

it is evident that a plurality of thyristor pairs could be included andconnected to various transformer taps depending upon the degree ofcontrol required. The more thyristor pairs provided the less waveformdistortion will be produced. It will also be seen that load transformer14 is necessary in the present situation only because of the voltagesand currents being controlled. Recognizing the voltage limitations ofthe thyristors, it was necessary that they be operated at a restrictedvoltage rating while the load was operated at a higher voltage. If thiswere not the case, transformer 14 could have been omitted. It will alsobe understood that the specific circuitry for producing the pulses,while preferred, is not the only possible arrangement.

It is also noted that the square law device designated 21 which producesa signal proportional to the square of the current in currenttransformer 19 may take various forms. For example, it may be producedby a proper arrangement of a Hall generator. Integrated devices areavailable however which will produce a square law signal in response toan input signal.

Finally, it is noted that the whole apparatus is as sumed to be operatedon a 60-cycle alternating supply, and while it is necessary that thevarious supplies all be from the same source, there is no reason why itshould' not operate at some other frequency dependent on the availablesource of supply.

rent having at least two secondary taps, means to selectively connect aload to one of said taps during portions of the cycle and therebyregulate the current supplied to said load, said means including a pairof oppositelypoled thyristors connected "to each tap, means to produce afirst signal representative of the square of the currents supplied tothe load, means to produce a second signal representative of the desiredload current, means to compare said first and second signals to producean error signal, means to integrate said error signal, means to comparesaid integrated error signal with a first reference signal and produce afirst output signal when said error signal exceeds said first referencesignal, a source of cyclic sawtooth waveforms in phase synchronism withthe alternating current being regulated, means to combine said outputsignal with said sawtooth waveform and produce a first control signalwhen said sawtooth waveform is equal in absolute value to said outputsignal, a first oscillator rendered operative by said first controlsignal, means to apply the out put of said first oscillator to thefiring electrodes of a first of said pairs of oppositely-poledthyristors in the proper phase to cause the proper one of said firstpair of thyristors to conduct and connect a first tap to said load,means to compare said integrated error signal with a second referencesignal and produce a second output signal when said error signalexceedssaid second reference signal, means to produce a second control signal,a second oscillator rendered operative by said second control signal andmeans to apply the output of said second oscillator to the firingelectrodes of a second of said pairs of oppositely-poled thyristors inthe proper phase to cause the proper one of saidsecond pair ofthyristors to conduct and connect a second tap to said load.

2. A regulation system as claimed in claim 1 wherein said secondreference signal is greater than said first reference signal and thevoltage supplied by said second tap is greater than the voltage appliedby the first tap. I

1. An alternating current regulation system comprising a supplytransformer supplied with alternating current having at least twosecondary taps, means to selectively connect a load to one of said tapsduring portions of the cycle and thereby regulate the current suppliedto said load, said means including a pair of oppositely-poled thyristorsconnected to each tap, means to produce a first signal representative ofthe square of the currents supplied to the load, means to produce asecond signal representative of the desired load current, means tocompare said first and second signals to produce an error signal, meansto integrate said error signal, means to compare said integrated errorsignal with a first reference signal and produce a first output signalwhen said error signal exceeds said first reference signal, a source ofcyclic sawtooth waveforms in phase synchronism with the alternatingcurrent being regulated, means to combine said output signal with saidsawtooth waveform and produce a first control signal when said sawtoothwaveform is equal in absolute value to said output signal, a firstoscillator rendered operative by said first control signal, means toapply the output of said first oscillator to the firing electrodes of afirst of said pairs of oppositely-poled thyristors in the proper phaseto cause the proper one of said first pair of thyristors to conduct andconnect a first tap to said load, means to compare said integrated errorsignal with a second reference signal and produce a second output signalwhen said error signal exceeds said second reference signal, means toproduce a second control signal, a second oscillator rendered operativeby said second control signal and means to apply the output of saidsecond oscillator to the firing electrodes of a second of said pairs ofoppositely-poled thyristors in the proper phase to cause the proper oneof said second pair of thyristors to conduct and connect a second tap tosaid load.
 2. A regulation system as claimed in claim 1 wherein saidsecond reference signal is greater than said first reference signal andthe voltage supplied by said second tap is greater than the voltageapplied by said first tap.